U.S. patent number 5,712,982 [Application Number 08/531,105] was granted by the patent office on 1998-01-27 for tdma point-to-multipoint transmission network with a multiframe which includes a single continuous stream of data subframes and a single free period for response-time measurements.
This patent grant is currently assigned to Alcatel Cit. Invention is credited to Fran.cedilla.ois Marcel.
United States Patent |
5,712,982 |
Marcel |
January 27, 1998 |
TDMA point-to-multipoint transmission network with a multiframe
which includes a single continuous stream of data subframes and a
single free period for response-time measurements
Abstract
In a time division multiple access point-to-multipoint
transmission network, data is transmitted from a central station to
local stations in multiframes containing a plurality of frames and
a single free period allowing transmit time measurements and
reflectometry measurements. Data is transmitted from the local
stations to the central station in the form of multiframes
containing a plurality of frames and a single free period allowing
transmit time measurements and reflectometry measurements, each
frame including subframes sent by the local stations, and each
local station sending one or more of the subframes contained in a
frame. The multiframes are constructed in a way that underuses the
data sources, to obtain free periods between the frames, and by
time-shifting the frames to construct a single free period per
multiframe. Applications include telecommunication networks.
Inventors: |
Marcel; Fran.cedilla.ois
(Orsay, FR) |
Assignee: |
Alcatel Cit (Paris,
FR)
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Family
ID: |
9467234 |
Appl.
No.: |
08/531,105 |
Filed: |
September 20, 1995 |
Foreign Application Priority Data
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Sep 23, 1994 [FR] |
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94 11401 |
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Current U.S.
Class: |
709/236; 370/442;
370/503; 370/508; 370/510; 709/231; 709/234 |
Current CPC
Class: |
H04J
3/0682 (20130101) |
Current International
Class: |
H04J
3/06 (20060101); G06F 013/372 (); G06F 013/376 ();
H04L 005/22 () |
Field of
Search: |
;370/462,362,249,250,257,503,442,508,510
;395/200.17,200.13,200.19 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0229684 |
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Jul 1987 |
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EP |
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WO8805233 |
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Jul 1988 |
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WO |
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WO9222151 |
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Dec 1992 |
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WO |
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WO9319540 |
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Sep 1993 |
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WO |
|
Primary Examiner: Lee; Thomas C.
Assistant Examiner: Kim; Ki
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak &
Seas, PLLC
Claims
What is claimed is:
1. Time division multiple access point-to-multipoint transmission
network including a central station and a plurality of local
stations;
each local station including means for sending return multiframes
including a plurality of return frames and a single free period
having a duration allowing measurements to be made, each return
frame including a plurality of return subframes each having a fixed
duration and containing data sent by that local station only and
addressed to said central station, return subframes being sent at
predetermined times decided on by transmit time measurement means
so that subframes sent by all said local stations are received by
said central station without overlapping and constitute a return
frame;
said central station including means for sending forward
multiframes including a plurality of forward frames and a single
free period having a duration allowing measurements to be made;
wherein, in said central station and/or at least one local station,
said means for sending multiframes include:
means for sampling from a data source a series of data packets, two
consecutive packets being separated by a free period, and for
inserting each packet into a frame, two consecutive frames being
separated by a free period; and
time-shift means for inserting a predetermined number of frames
into a multiframe, time-shifting said frames to eliminate the free
periods between them and thus to constitute a multiframe including
a single continuous stream of time-shifted frames and a single free
period.
2. The transmission network of claim 1 wherein said time-shift
means include sufficient memory for storing at least a number of
bits corresponding to said single free period.
3. The transmission network of claim 1 wherein said time-shift
means include a dual-port memory having an input clock and output
clock, said memory input clock is disabled for each said free
period between said consecutive frames, and said memory output
clock is periodically disabled for a duration equivalent to said
single free period accumulated from a predetermined number of said
free periods between said consecutive frames.
4. The transmission network of claim 3 wherein said memory further
comprises a FIFO including: a serial-to-parallel converter, a
parallel-to-serial converter, and control means for controlling
said converters and memory such that an input bit stream is
converted to a parallel stream, stored into and read from said
dual-port memory, and then converted back to an output bit stream
according to said input clock and said output clock.
5. A time division multiple access point-to-multipoint transmission
network including a central station and a plurality of local
stations connected to a common transmission path;
said central station including means for sending forward frames,
said forward frames each including a single free period having a
duration allowing response-time measurements to be made;
each local station including means for sending return frames, said
return frames each including a plurality of subframes and a single
free period having a duration allowing response-time measurements
to be made;
wherein each subframe is sent by a respective local station at a
predetermined time relative to a frame start time, said
predetermined time being adjusted according to said response-time
measurements for said each respective local station; and
wherein said means for sending forward frames and means for sending
return frames include:
means for building a shift frame comprising a single free period
and a plurality of input data subframes clocked from a data source
with free periods inserted between each two consecutive input data
subframes; and
time-shift means for storing a predetermined number of bits of
consecutive shift frames, accumulating the free period of each said
shift frame, and generating a multiframe including a single
continuous stream of time-shifted shift frames and a single
accumulated free period.
6. The transmission network of claim 5 wherein said time-shift
means include a FIFO memory having an input clock and an output
clock, and sequentially storing a predetermined number of bits of
said shift frames using said input clock, said output clock being
enabled for a predetermined number of said shift frames, and then
disabled for a time corresponding to said predetermined number of
shift frames.
7. The transmission network of claim 6 wherein said predetermined
number of bits is determined by the free period in each shift
frame.
8. The transmission network of claim 6 wherein said FIFO memory
includes sufficient memory for storing at least a number of bits
corresponding to said accumulated free period.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention concerns a time division multiple access
point-to-multipoint transmission network including a central
station and a plurality of local stations.
2. Description of the Prior Art
In a network of this kind the central station sends forward frames
containing data addressed to the local stations, each forward frame
having a fixed duration and including common data addressed to all
local stations or separate data for respective separate local
stations. Each local station sends return subframes each having a
fixed duration and including data sent only by that local station
and addressed to the central station. The local stations send these
subframes in such a way that they do not overlap in time and
constitute a return frame.
As the local stations are at different distances from the central
station, the transmit time between a local station and the central
station is not the same for all the stations. Means for measuring
the transmit times determine the times for each local station to
transmit so that the subframes sent by all the local stations are
received by the central station without overlapping. A known way to
measure transmit times involves the central station and the local
stations ceasing to send data during periodic time periods.
For example, European patent application EP-A-0 188 117 describes a
time division multiple access network in which:
a central station sends frames having a duration of 125
microseconds including a plurality of subframes followed by an
address addressed to one of the local stations and telling it to
send a test message, followed by a free period for measuring the
transmit time; each subframe contains data addressed to a separate
local station;
the local stations each send in turn a subframe containing data to
be transmitted from that local station to the central station.
Each local station includes transmit time measuring means which
determine the transmit time between that local station and the
central station and deduce therefrom the times at which the local
station can send a subframe, so that the central station received a
frame made up of successive subframes that do not overlap.
This prior art document describes a method of measuring transmit
times that entails:
the central station sending a local station an address designating
that local station and commanding it to send a test message, this
address being inserted into a forward frame after the data
subframes and before the free period;
that local station sending the central station a test message as
soon as the local station has recognized its address, the free
period in the forward frames and the free period in the return
frames each having a duration sufficient for the test message
always to occur within these periods regardless of the distance to
a local station; this is in order to avoid collision with data;
reception of the test message at the central station followed by
its immediate retransmission in the free period of a forward
frame;
reception of the test message in the local station that sends it,
determination of the transmit time between the local station and
the central station, and deduction from the latter of times at
which that local station can send subframes.
This method requires a free period in the forward frames and a free
period in the return frames each having a duration at least equal
to the round trip transmit time plus the duration of the test
message; this is to prevent collisions between a test message and
data transmitted by the central station or by other local
stations.
In the example described in this prior art document the local
stations are connected to a private telephone exchange and are
distributed within the bounds of a building. The round trip
transmit time is therefore in the order of 10 microseconds. The
frames have a fixed duration of 125 microseconds, which corresponds
to the sampling period of a standard telephone channel in a digital
telephone network. It is possible to leave a free period of about
10 microseconds in a 125 microsecond frame without seriously
degrading the transmission performance of the network. A free
period of this duration is entirely satisfactory for a network
restricted to one building.
However, this method cannot be used in a network covering greater
distances, such as a public telecommunication network. A network of
this kind is connected to local stations at much greater distances,
possibly up to 10 km. The round trip transmit time then reaches
values in the order of 100 microseconds. It is not possible to
sacrifice the major part of the frame duration to provide a free
period having a duration in the order of 100 microseconds.
The prior art includes a transmit time measuring method that can
accommodate a free period of very much shorter duration than the
round trip transmit time. This method is tolerant of collisions
that may occur between the test messages and data. It is more
complex, however. It includes a step of approximate estimation of
the transmit time followed by a step of more refined
estimation.
The prior art further includes methods of testing a transmission
medium in a network of this kind using reflectometry, but these
methods usually entail interrupting operation of the network since
they require a free period that can be as much as twice the round
trip transmit time.
Document EP-A-0 229 684 describes a time division multiple access
point-to-multipoint transmission network in which data is
transmitted from a central station to local stations in multiframes
including a plurality of frames addressed to respective local
stations. Each multiframe includes a single free period for
transmit time measurements and reflectometry measurements. Data is
transmitted from local stations to a central station in the form of
multiframes including a plurality of frames and a single free
period for transmit time measurements and reflectometry
measurements, each frame including subframes sent by respective
local stations, each local station sending one or more of the
subframes contained in a frame. The free period in each multiframe
can have a duration equal to twice the maximal round trip transmit
time without seriously degrading the transmission performance of
the network, since the free periods occur much less frequently than
if there were to be a free period in each frame.
This prior art document also describes means for forming
multiframes with a single free period in each multiframe. These
means apply temporal compression to the data to be transmitted.
An object of the invention is to propose a point-to-multipoint
transmission network in which the central station and the local
stations include means for sending multiframes including a single
free period using a method other than the temporal compression
method.
SUMMARY OF THE INVENTION
The invention consists in a time division multiple access
point-to-multipoint transmission network including a central
station and a plurality of local stations;
each local station including means for sending return multiframes
including a plurality of return frames and a single free period
having a duration allowing measurements to be made, each return
frame including a plurality of return subframes each having a fixed
duration and containing data sent by that local station only and
addressed to the central station, return subframes being sent at
predetermined times decided on by transmit time measurement means
so that subframes sent by all the local stations are received by
the central station without overlapping and constitute a return
frame;
the central station including means for sending forward multiframes
including a plurality of forward frames and a single free period
having a duration allowing measurements to be made;
wherein, in the central station and/or at least one local station,
the means for sending multiframes include:
means for sampling from a data source a series of data packets, two
consecutive packets being separated by a free period, and for
inserting each packet into a frame, two consecutive frames being
separated by a free period; and
time-shift means for inserting a predetermined number of frames
into a multiframe, time-shifting said frames to eliminate the free
periods between them and thus to constitute a multiframe including
a single continuous stream of time-shifted frames and a single free
period.
The invention will be better understood and other features will
emerge from the following description and the appended
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 represents the block schematic of one example of a network
in accordance with the invention.
FIG. 2 is a schematic representation of a forward multiframe and
one of the frames constituting it.
FIG. 3 a schematic representation of a return multiframe, one of
the frames constituting it and a subframe constituting one of those
frames.
FIG. 4 represents a timing diagram illustrating the operation of
the means for sending multiframes in a manner that underuses the
capacity of a data source but retains its bit rate.
FIG. 5 represents the block schematic of one embodiment of these
means.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 represents the block schematic of one example of a network
in accordance with the invention. In this example the central
station sends data addressed to all the stations. In other examples
the central station could send different data to different local
stations, by allocating each frame of a multiframe to a given local
station.
The central station CS communicates with a public switched network
PSN and with local stations LS1, LS2, LS3, . . . , LSs via a
passive optical network PON conveying signals in both directions.
The central station CS transmits data addressed to all the local
stations LS1, . . . , LSs and the latter each transmit data to the
central station CS. The central station CS exchanges signals with
the public switched network PSN in the form of asynchronous
transfer mode cells at an average rate of one cell every 125
microseconds, or a multiple of this rate, which corresponds to the
standard period for sampling a telephone channel in digital
telephone networks. Each local station LS1, . . . , LSs exchanges
data with a terminal (not shown), for example a multimedia
microcomputer, at an average rate of one cell every 125
microseconds, for example.
The local stations LS1, . . . , LSs are at various distances from
the central station CS. The remotest local station LS1 is at a
distance DM in the order of 10 km, which implies a round trip
transmit time in the order of 100 microseconds. To allow transmit
time measurements when a station connects to the network and to
allow reflectometry measurements to monitor the state of the links
without collisions between the measurement signals and the data it
is necessary to interrupt the sending of data in both directions
during time periods each having a duration of at least 100
microseconds.
FIG. 2 shows the structure of a forward multiframe MF sent by the
central station CS in accordance with the invention. To simplify
the implementation of the network, each multiframe MF has a
duration TMF that is equal to a multiple of the telephone channel
sampling period. In this example:
where n.times.125 .mu.s is the duration of each frame F1, . . . ,
FN constituting the multiframe, n is an integer and N is the number
s of frames contained in a multiframe. The number N can be equal to
the number of local stations, but this is not essential. The frames
F1, . . . , FN are followed by a free period DMW having a fixed
duration TW. In this example, TW is equal to 121.81 .mu.s ,
allowing transmit time measurements and reflectometry measurements
for a maximal distance DM in the order of 10 km; n is equal to 1
and the fixed duration of each frame is TF=121.19 .mu.s, which is
slightly less than the telephone channel sampling period.
The content of each frame depends on the intended application. If
the central station sends different data to different local
stations, each frame (for example frame Fj) is divided into
subframes (not shown) addressed to different local stations LS1, .
. . , LSs. If the central station CS sends coon data to all the
local stations the data can be distributed in any way within the
frames F1, . . . , FN. In this latter case, the frame F1 includes a
multiframe header MFH and a payload PL1 but all the other frames
F2, . . . , FN contain only a payload. The header MFH is used for
administration of the data transmitted in the payload of all the
frames F1, . . . , FN constituting the multiframes MF. In one
implementation in which the bit rate of the forward multiframes is
155.52 Mbit/s, each frame includes 2356 bytes, the first frame F1
including a header MFH of 16 bytes and a payload PL1 of 2340
bytes.
FIG. 3 shows the structure of a forward multiframe MF'. In this
example, it has a fixed duration:
where N' is the number of frames F'1, . . . , F'N' in the
multiframe MF'and n'.times.125 .mu.s is the duration of each of
these frames, n' being an integer. These frames are followed by a
free period DMW' having a duration TW'=177.92 .mu.s allowing
transmit time measurements and reflectometry measurements. The
duration of this free period being very much greater than 100
microseconds, it allows transmit time measurements for a maximal
distance DM greater than 10 km.
Each frame has a fixed duration TF'=119.44 .mu.s and in this
example is made up of 43 subframes SF1, . . . , SFi, . . . , SF43.
Each local station LS1, . . . , LSs sends one or more subframes
according to the quantity of data that it has to send, the
allocation of the subframes being fixed for the duration of a call.
The time at which a station sends its subframe or subframes is
determined by transmit time measuring means so that the subframes
SF1, . . . , SF43 reach the central station CS without overlapping
and constitute a continuous data packet.
In this example each subframe, for example subframe SFi, is made up
of a header with a fixed length of 1 byte, including a guard space,
and a payload Ci1 made up of 53 bytes which can be samples of a
plurality of standard telephone channels or the 53 bytes BY1, . . .
, BY53 of an asynchronous transfer mode cell. For example, if a
local station LSi receives from a data source a sequence of
successive cells Ci1 , . . . , CiN', . . . , at the average rate of
one cell every 125 .mu.s, these cells are transported in respective
frames F'1, . . . , F'N' of the multiframe MF'.
This transmission network achieves good transmission performance
despite the existence of free periods to allow for transmit time
measurements having durations in the order of 100 .mu.s, since
these free periods are repeated at multiples of the duration of the
frame, which reduces the relative size of the free periods compared
to the data transmitted. For example, if each multiframe is made up
of 32 frames each having a duration of 125 microseconds, and if the
free period duration is 100 microseconds, the free periods occupy
only 2.4 % of the total time.
These free periods have a duration enabling them also to be used to
test the status of the network transmission medium, by
reflectometry.
The method used to form the forward multiframes MF and to form the
return multiframes MF' entails underusing the capacity of each data
source, i.e. requiring it to supply fewer data packets than it
could supply during a multiframe.
FIG. 4 represents a timing diagram illustrating this underuse of
the capacity of a data source. The data source has a capacity such
that it could supply an uninterrupted stream of bytes at a fixed
bit rate H. In fact it is used to supply frames F1c, F2c, F3c, . .
. , FNc including n-m bytes, two consecutive frames being separated
by a free period corresponding to bytes. A multiframe is
constructed by concatenating N frames F1c, . . . , FNc to
constitute an uninterrupted stream of frames F1d, . . . , FNd at
the same bit rate H but time-shifted relative to the frames F1c, .
. . , FNc so as to combine into a single free period the N free
periods each corresponding to m bytes.
The free period chosen for the multiframe corresponds to q bytes,
with q=N.times.m, and consequently the choice of the duration of
the free period in the multiframe determines the number of bytes
corresponding to the free period between two successive frames
supplied by the source. The underuse of the source can be
represented by the ratio m/n.
FIG. 5 represents the block schematic of one embodiment of means
for sending multiframes that underuse the capacity of a data source
and retain its bit rate. This device can be used to send
multiframes either from a central station to the local stations or
from a local station to a central station.
This embodiment includes: a conventional frame builder device 31
having an input 16 connected to a data source and an output
supplying frames F1c, . . . , FNc, each including n-m bytes, at a
bit rate H, two consecutive frames being separated by a free period
corresponding to bytes, and a time-shift First In, First Out (FIFO)
memory device 30 for forming multiframes.
The device 30 includes:
an input 20 receiving a clock signal H at the bit rate of the data
in frames F1c, . . . , FNc;
an input 21 receiving a synchronization signal S at the multiframe
bit rate;
an input 22 receiving the data frames F1c, . . . , FNc in serial
form;
an output 29 supplying a multiframe made up of frames F1d, . . . ,
FNd and a free period corresponding to 9 bytes;
a frequency divider 23 that divides by eight and has a clock input
connected to the input 20, a synchronization input connected to the
input 21 and three outputs;
a serial-parallel converter 26 having a data input connected to the
input 22, a first clock input connected to the input 20, a second
clock input connected to a first output of the divider 23 supplying
a clock signal at the bit rate H/8 of the bytes of the frames F1c,
. . . , FNc, and one output;
a dual port random access memory 27 having a data input connected
to the output of the device 26, a write command input, a read
command input and an output;
a parallel-serial converter 28 having a data input connected to the
output of the memory 27, a first clock input connected to the input
20, a second clock input connected to a second output of the
divider 23 supplying a clock signal at the bit rate H/8 which is
that of the bytes in frames F1d, . . . , FNd, and an output
connected to the output 29;
a write clock 24 having a clock input connected to a third output
of the divider 23 and supplying a clock signal at the bit rate H/8
of the bytes in frames F1c, . . . , FNc, a synchronization input
connected to the input 21 and an output connected to the write
command input of the memory 27;
a read clock 25 having a clock input connected to the second output
of the divider 23 supplying a clock signal at the bit rate H/8, a
synchronization input connected to the input 21 and an output
connected to the read command input of the memory 27.
The memory 27 has a capacity corresponding to at least .alpha.
bytes, the latter being stored byte by byte. The converter 26
stores the data bits from frames F1c, . . . , FNc in the form of
bytes. The converter 28 converts to serial form the data read byte
by byte from the memory 27. For each frame F1c, . . . , FNc the
write clock 24 commands the writing of n-m bytes at the bit rate
H/8 and then commands no writing during a time period corresponding
to bytes. For each multiframe the read clock 25 commands no reading
for a time period corresponding to .alpha. bytes and then commands
p-q reads of one byte at the bit rate H/8. The frames F1d, . . . ,
FNd are therefore time-shifted relative to the respective frames
F1c, . . . , FNc with time-shifts such that the free periods m
between these frames are eliminated and such that the frames F1d, .
. . , FNd form a continuous stream of bits and are preceded by a
single free period.
Prior art transmit time measurement and prior art reflectometry
measurement means can be used in a network in accordance with the
invention.
The invention can be applied to any time division multiple access
point-to-multipoint network and in particular those in which the
data sources are synchronous or plesiochronous.
* * * * *